Puller cell

ABSTRACT

A crystal puller cell (18) provides a low particulate environment for an individual crystal puller (28). The airflow within each cell is adjustable so that a particulate level appropriate to the activity within the cell is maintained, thereby avoiding the cost of maintaining an entire growing hall (10) at a constant high level of cleanliness. Each cell includes a multilevel floor (46) that includes an operator floor (48) and a maintenance floor (52). A door (62) at the maintenance floor level opens onto a maintenance aisle used to service the machines. A door (64), at the operator floor level, opens onto a clean aisle for transporting raw material and finished product. The cell walls can include magnetic shielding if a magnetic growing process is used to reduce exposure of operators and other machines to intense magnetic fields.

TECHNICAL FIELD

This invention relates to facilities for growing crystals for use in thesemiconductor industry and, in particular, to a crystal puller cell forhousing a Czochralski crystal puller and to a growing hall housingmultiple ones of such cells.

BACKGROUND OF THE INVENTION

Integrated circuits are typically fabricated on wafers of asingle-crystal semiconductor material. The wafers are sliced from asingle crystal ingot grown in a Czochralski-type crystal growingmachine, referred to as a "crystal puller." Typically, many crystalpullers are located within a single large room, called a "growing hall."A control console located by each crystal puller allows a crystal pulleroperator to control each crystal puller in accordance with instrumentreadings and observations through an observation window of conditionswithin the crystal puller.

To grow an ingot, a seed crystal is dipped into a crucible of moltensemiconductor and then slowly retracted as the seed and the crucible arerotated in opposite directions. The semiconductor freezes onto the seedas it is retracted to produce a single crystal ingot. Crystals aresometimes grown in the presence of a strong magnetic field which, bycontrolling thermal convection in the molten semiconductor, produces aningot having a more uniform amount of oxygen incorporated into the ingotfrom the crucible walls.

The growing process begins with loading into a crucible a "charge" ofultrapure polysilicon. A dopant is often added to the charge to changethe electrical properties of the resultant crystal. A seed crystalhaving the desired crystal orientation is then secured within thecrystal puller in a chuck attached to a cable that is used to raise theseed, and a charged crucible is loaded into the crystal puller. Thesection of the crystal puller in which the seed crystal and the chargeare placed is called the "furnace tank." As the single-crystal ingot isgrown, it is received into a "pull chamber" above the furnace tank. A"mechanical unit" below the furnace tank includes motors and othermechanical and electrical devices used in the crystal growing process.

Before melting the charge, the air in the furnace tank and pull chamberis evacuated and replaced with an inert gas, such as argon. The crucibleis rotated as the charge within it is melted, typically using aresistant heater. The seed crystal is dipped slightly into the meltedcharge and slowly retracted to grow the ingot. After the ingot is grownand cooled, the pull chamber is opened and the ingot is removed.

Extremely small amounts of contamination in a crystal can have severeadverse effects on the characteristics of electronic circuits fabricatedon the crystal. It is critical, therefore, that airborne contaminationbe minimized in the growing area. Although clean room techniques forreducing airborne particulate contamination are widely used in thesemiconductor industry, it is difficult and expensive to maintain a highdegree of cleanliness in a large growing hall. The energy cost ofproviding the required volume of filtered airflow is too great. Higherthan optimum particulate levels have, therefore, been tolerated ingrowing halls, along with corresponding contamination of the growncrystals.

Cross-contamination problems are much worse during the installation ofnew equipment. Growing halls typically include an overhead bridge cranethat is used to install new equipment and repair existing equipment. Useof such an overhead crane produces particulate contamination thatsettles down onto the active crystal pullers.

To reduce contamination of the crystals during the installation of newcrystal pullers, one practice has been to install several machines atone time in a segregated area of the growing hall. Under thesecircumstances, the growing hall is not being continually contaminatedand disrupted by the installation of individual machines.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide acrystal-growing environment having a reduced level of airborneparticulate contamination.

Another object of this invention is to provide such an environment at areasonable cost and to achieve such an environment at a minimum runningcost.

A further object of this invention is to provide such an environmentcapable of modular expansion.

Yet another object of this invention is to provide such an environmentthat is capable of reducing exposure of machine operators to magneticfields.

Still another object of the present invention is to improve theproductivity of crystal puller operators.

The present invention is an economical, low contamination environmentfor a crystal puller. The environment includes a crystal-puller cell forhousing a crystal puller and a growing hall containing multiple crystalpuller cells.

The crystal puller cell comprises plural cell walls that isolate theenvironment within the cell from the environment in the growing halloutside the cell and a base floor at a first level for mounting of acrystal puller. Above the base floor is a second floor for accessing thecrystal puller. The second floor is preferably a multi-level floorhaving a second, or operator, level positioned at a height convenientfor loading the raw material and unloading the single crystal and anintermediate, or maintenance, level positioned at a height between thefirst and second levels and convenient for maintaining the crystalpuller. If a crystal puller cell houses a crystal puller using amagnetic process, the walls of the cell can be shielded to reduceexposure of workers and other equipment to the intense magnetic fields.

The crystal puller cell also includes an adjustable air contaminationcontrol system for maintaining airborne particulate within the cellbelow specified levels. The particulate level specified at anyparticular time is appropriate for the operation being performed withinthe crystal-growing cell. For example, the specified particulate levelwould be lowest, i.e., the environment within the cell would becleanest, when the semiconductor material is exposed to the environment,i.e., when the raw material is being loaded and the single crystal isbeing unloaded. A higher particulate level can be tolerated when thecrystal puller is operating and the furnace tank and pull chamber areclosed. An even higher particulate level could be tolerated when themachine is idle. By maintaining only the level of cleanliness required,the costs associated with maintaining suitable particulate levels aregreatly reduced.

The area within the growing hall is divided into three types of areas:puller cells, clean aisles, and maintenance aisles. Each area has anenvironment characterized by an airborne particulate level, whichcorresponds to a level of cleanliness. A relatively high degree ofcleanliness is maintained in the clean aisles, which are used to bringraw material to and finished products from the puller cell. A lowerdegree of cleanliness is maintained in the maintenance aisles, which areused by maintenance workers to maintain the crystal pullers.

Each crystal puller cell has two doors: a first door, at the operatorlevel, that opens onto a clean aisle and a second door, at themaintenance level, that opens onto a maintenance aisle. Crosscontamination is prevented by logic circuitry that electronicallycontrols the door locks to prevent both doors from being openedsimultaneously and to prevent either door from opening when theenvironment within the puller cell is not compatible with that outsidethe door.

By maintaining separate environments in areas in which the semiconductormaterial is exposed to the environment, a low particulate level can bemaintained in appropriate areas at a lower overall cost. Providing aseparate environment for each crystal puller not only isolates eachpuller from contamination in the growing hall, it allows a reduction inairflow when appropriate in each cell, thereby further reducing costs.Moreover, additional crystal puller cells and crystal pullers can beinstalled with minimal contamination of existing machines.

In accordance with another aspect of the invention, a remote controlconsole capable of controlling multiple crystal pullers is located in aclean aisle away from the doors of the puller cells. By observinginstruments and video images transmitted to the remote control panelfrom a television camera positioned to observe conditions within thecrystal puller, an operator can operate the multiple crystal pullersfrom one remote location, thereby reducing contamination in theimmediate vicinity of the pullers, reducing operator exposure tomagnetic fields, and improving productivity by allowing a singleoperator to operate more machines.

Additional objects and advantages of the present invention will beapparent from the following detailed description of a preferredembodiment thereof, which proceeds with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a growing hall designed in accordance with thepresent invention.

FIG. 2 is a cross sectional view of a crystal puller cell taken alongline 2--2 of FIG. 1.

FIG. 3 shows a prefabricated ceiling unit used in the crystal pullercell of FIG. 2.

FIG. 4 is a block diagram of a control unit used to control the pullercell of FIG. 2.

FIG. 5 is a schematic view of a remotely controlled bay includingseveral of the crystal puller cells of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the layout of a crystal-growing hall 10 of the presentinvention. Hall 10 is contained within a shell 12 formed from multiplewalls 14 and a high ceiling 16 (FIG. 2) that provides an unobstructed,column-free expanse for containing multiple crystal puller cells 18 ofthe present invention. FIG. 2 shows a cross sectional view of apreferred crystal puller cell 18 of the present invention withincrystal-growing hall 10. The environment within crystal puller cell 18is isolated from the environment within hall 10 by prefabricated walls24 and a prefabricated ceiling unit 26. Crystal puller cell 18 houses amulti-story crystal puller 28, which rests upon a prefabricated concretepad 30, which, in turn, rests upon a base floor 32.

The use of prefabricated components to construct puller cell 18eliminates the need for heavy construction in growing hall 10, therebyreducing sources of contamination. The use of prefabricated componentsalso reduces the time required to construct puller cell 18.

If a magnetic process is used in crystal puller machine 28,prefabricated walls 24 can include a magnetic shielding material, suchas low carbon, mild steel, or other appropriate metal alloy to preventexposure of machine operators and other machines to intense magneticfields. Moreover, physically separating an exemplary puller cell 18afrom the other puller cells 18 can further reduce the propagation ofmagnetic fields from puller cell 18a through the metal walls to theother puller cells 18.

Crystal puller 28 comprises a pull chamber 42a into which the grownsingle crystal is received, a furnace tank 42b in which a semiconductormaterial is melted and grown into a single crystal, and a mechanicalunit 44, positioned below furnace tank 42b, that contains mechanical andelectrical components that operate crystal puller machine 28.

Puller cell 18 also includes a discontinuous multi-level floor 46comprising an operator floor 48 positioned at an operator levelconvenient to pull chamber 42a and a maintenance floor 52 at amaintenance level convenient to furnace tank 42b. Operator floor 48 istypically level with the bottom of pull chamber 42a so that an operatoror a material handling device (not shown) can easily access the pullchamber 42a. Such an operator floor 48 is particularly useful withcrystal pullers 28 that use a door-type pull chamber 42a. It will beunderstood that ingots grown in crystal puller 28 can be quite large,some being longer than two meters and weighing over one hundredkilograms. Access to crystal puller 28 is provided through two doors inpuller cell 18, a maintenance door 62 that opens onto the maintenancelevel and an operator door 64 that opens onto the operator level.

FIG. 3 shows that prefabricated ceiling unit 26 includes a filterhousing 78 for housing high-efficiency particulate air ("HEPA") filters80, lighting fixtures 82, and fire-prevention sprinklers 84 all mountedon a common ceiling grid 86. Grid 86 is mounted at the top of walls 24,above crystal pulling machine 28 and below a hall ceiling 16, andattached to a plenum 88 to provide air flow. Electric power is suppliedto grid 86 to provide lighting within puller cell 18, and grid 86 isattached to a water supply for use by fire-prevention sprinklers 84.Because ceiling units 26 are prefabricated, the amount of heavyconstruction required in growing hall 10 is reduced. By mounting ceilingunits 26 on walls 24 of an appropriate height, puller cell 18 canaccommodate crystal puller machines 28 of various heights. Free ceiling16 permits the use of walls 24 of various heights, thereby providingflexibility in constructing within a single growing hall 10 puller cells18 that accommodate machines of different sizes, including largermachines that may be required in the future.

A bridge crane 94 is typically installed below building trusses 92 andis used in the installation and repair of crystal pullers 28 and in theconstruction of puller cells 18. Bridge cranes are inevitably a sourceof particulate contamination but, because bridge crane 94 is aboveceiling unit 26, particles shed by bridge crane 94 do not contaminatethe environment within crystal puller cell 18.

Filtered air is supplied to HEPA filters 80 in ceiling unit 26 by plenum88. The airflow within each puller cell 18 is individually controlled byairflow control circuitry 98 within a control unit 100 (FIG. 4), airflowcontrol circuitry 98 controlling an air supply system comprisingvariable airflow dampers 104 and a variable frequency motor 106 in anair handler unit 102 to produce an appropriate airflow at any particulartime. Airflow control circuitry 98, an air handler unit ("AHU") 102,variable air flow dampers 104, an airborne particle counter 131, andHEPA filters 80 together comprise an adjustable air contaminationcontrol system 108.

In one embodiment, a low airflow mode is used when puller cell 18 isidle or is being cleaned. A moderate airflow rate is used when crystalpuller 28 is operating with the pull chamber 42a and furnace tank 42bclosed. A high airflow rate is used when maximum cleanliness isrequired, i.e., when raw material is being loaded or a single crystal isbeing unloaded. A purge airflow rate, which is greater than the highairflow rate, is used to purge the interior of puller cell 18 after ithas been contaminated. Each airflow operation made at each puller cell18 is controlled by a status signal from a controller 29 of crystalpuller 28 and by an airborne particle counter 131 or other device whichindicates the particulate level in the puller cell.

Referring to FIG. 1, the area within growing hall 10 is divided intopuller cells 18, clean aisles 114, and maintenance aisles 116. Pullercell maintenance door 62 opens onto maintenance aisle 116, and operatordoor 64 opens onto clean aisle 114. A relatively high degree ofcleanliness is maintained in clean aisles 114, which are used to bringraw materials and finished products to and from puller cell 18. The rawmaterial typically comprises ultrapure polycrystalline silicon,precharged in a quartz crucible; the finished product is typically asingle crystal silicon ingot. A lower degree of cleanliness ismaintained in maintenance aisle 116, which is used by maintenanceworkers to maintain crystal puller 28. To maintain their cleanliness,clean aisles 114 have clean aisle ceilings 118 (FIG. 2) at a suitableheight 120 with particulate filters 121 that continuously providefiltered air to clean aisles 114. Ceiling height 120 is typically muchlower than that of a prior art growing hall, so less airflow is neededto maintain the required level of cleanliness.

Control unit 100 includes logic circuitry 122 that controls locks 124and 126 on respective doors 62 and 64 to eliminate cross contaminationamong puller cell 18, clean aisle 114, and maintenance aisle 116. Logiccircuitry 122 prohibits opening either door when conditions withinpuller cell 18 do not match those outside that door. For example, ifpuller cell 18 is operating in a high flow mode to obtain a high degreeof cleanliness or if puller cell 18 is operating in a moderate airflowmode to obtain a medium degree of cleanliness, logic circuitry 122 willprevent first door 62 from opening onto maintenance aisle 116.Similarly, if puller cell 18 is operating in a low flow mode and firstdoor 62 is opened, logic circuitry 122 will prevent second door 64 fromopening onto clean aisle 114. Logic circuitry 122 also prevents firstand second doors 62 and 64 from being opened simultaneously.Furthermore, airborne particle counters 131 monitor particle level inpuller cells 18. Logic circuitry 122 has a data interface with airborneparticle counter 131 and prevents second door 64 from opening onto cleanaisle 114 unless the particle level in puller cell 18 is compatible withthat of clean aisle 114. Also, logic circuitry 122 prohibits openingeither door while a magnet device in the puller cell is energized. Thisfeature can keep operators away from areas in which a strong magneticfield is present.

FIG. 5 shows schematically a remote control console 136 for controllingseveral puller cells 18 within a bay 138. Remote control 136 is locatedin clean aisle 114 and electrically connected by conductors 139 toindividual puller controller 29 through control consoles 140 positionedadjacent to each crystal puller cell 18. A television camera 142 locatedwithin each puller cell 18 transmits images of the conditions withineach crystal puller 28 to an operator, who can observe the images on avideo display by remote console control 136.

By allowing the operator to monitor multiple crystal pullers 28 from onelocation, the operator's productivity is greatly increased. Byeliminating the requirement for operators to move among the crystalpullers 28 to monitor the pulling operations, much of the particulatecontamination caused by the operators is eliminated. The operator isalso removed from magnetic fields used in any of the puller cells 18.Because remote control console 136 is located in clean aisle 114, theoperator can quickly proceed to an individual puller cell 18, whenremote control console 136 indicates such a visit is required.

By individually controlling the environment surrounding each crystalpuller machine 28, the present invention allows the areas in growinghall 10 to maintain appropriate cleanliness while reducing the overallfacilities cost. Because smaller areas are kept clean, the cost ofproviding the necessary flow of filtered and conditioned air to maintainthe cleanliness level is much less expensive, and those smaller areascan be maintained in a cleaner condition. Adjusting the airflow to alevel appropriate to the activity with puller cell 18 further reducescosts.

Additional crystal puller machines 28 can be added while previouslyinstalled machines continue operating without contamination, therebyallowing flexibility in scheduling installation of new machines. It ismore effective to invest capital in crystal pullers and puller cellsonly as required to meet production schedules.

It will be obvious that many changes may be made to the above-describeddetails of the invention without departing from the underlyingprinciples thereof. For example, a puller cell 18 could also accommodatetwo or more crystal pullers 28. Such an arrangement would increaseoperating costs, but reduce construction costs. Puller cell 18 could bedivided by an internal wall into a maintenance area, defined bymaintenance floor 52, and an operator area, defined by operator floor48, with the environments of the maintenance and operator areas beingseparately controllable. The scope of the present invention should,therefore, be determined only by the following claims.

We claim:
 1. A crystal-growing hall containing multiple crystal pullersand simultaneously providing multiple environments within the growinghall, each environment characterized by an airborne particulate level,the airborne particulate level in each environment being maintained at alevel appropriate to the activities that occur with the environment,comprising:shell walls defining the interior of the growing hall;multiple crystal puller cells located within the interior of the growinghall, each puller cell having an cell interior environment, the airborneparticulate in the cell interior environment being maintained at a levelappropriate to the activity within the crystal puller cell at aparticular time; a clean aisle maintaining an environment having arelatively low level of airborne particulate, the clean aisle connectingthe crystal puller cells with a source of raw materials and a place todeposit grown crystals; and a maintenance aisle within the growing hall,the maintenance aisle maintaining an environment having an airborneparticulate level higher than that of the clean aisle environment. 2.The crystal-growing hall of claim 1 in which each of the crystal pullercells includes a door that opens onto the clean aisle and a door thatopens onto the maintenance aisle, the doors being controlled by logiccircuitry that prevent cross contamination caused by opening both doorssimultaneously.
 3. The crystal-growing hall of claim 1 in which each ofthe crystal puller cells include a door that opens onto the clean aisleand a door that opens onto the maintenance aisle, the doors beingcontrolled by logic circuitry that prevents cross contamination causedby opening doors when the particulate level within the crystal pullercell is incompatible with the particulate level in the aisle on theother side of the door.
 4. The crystal-growing hall of claim 1, in whichthe low level of airborne particulate maintained in the clean aisle isthat of class 10,000 or better.
 5. The crystal-growing hall of claim 1in which the particulate level maintained in the maintenance aisle isthat of a class 100,000 or better.
 6. The crystal-growing hall of claim1 in which the crystal puller cells include magnetic shielding to reduceexposure of operators to intense magnetic fields and in which each ofthe crystal puller cells includes a door that opens onto the clean aisleand a door that opens onto the maintenance aisle, the doors beingcontrolled by logic circuitry that prohibits opening the doors while amagnet within the crystal puller cell is energized.
 7. Thecrystal-growing hall of claim 1 further comprising a remote controlconsole for operating a crystal puller within a crystal puller cell, theremote control console positioned outside of the puller cell andallowing a crystal puller operator to the crystal puller withoutentering the crystal puller cell.
 8. The crystal growing hall of 7further comprising a remote viewing instrument so that the crystalpuller operator can observe conditions inside the crystal puller.
 9. Thecrystal growing hall of 7 in which the remote control console allows thecrystal puller operator to operate a multiple crystal pullers from asingle location so that the productivity of the crystal puller operatoris improved.
 10. The crystal-growing hall of claim 1, wherein at leastone of the multiple crystal puller cells comprisesa first floorpositioned at a first level for supporting a crystal puller; a secondfloor for accessing the crystal puller.
 11. The crystal-growing hall ofclaim 10 in which the second floor of at least one of the multiplecrystal puller cells comprises a multi-level operations floor having asecond level and an intermediate level, the second level beingappropriate for loading raw material and unloading finished crystal andcorresponding to the level of the clean aisle and the intermediate levelbeing appropriate for maintaining the crystal puller and correspondingto the level of the maintenance aisle.
 12. The crystal-growing hall ofclaim 10 further comprising an adjustable air contamination controlsystem for maintaining airborne particulate within the cell below aspecified level, the specified level being adjustable to correspond tothe activity performed in the housing and being controllableindependently of the flow rate in the growing hall outside of the cell.13. The crystal-growing hall of claim 1, wherein the crystal pullercells opening onto the maintenance aisle to enable maintenance workersto maintain the crystal pullers.